Polyvinylidene fluoride is known as having a certain number of advantageous properties, particularly a relatively low melting temperature (about 170.degree. to 180.degree. C. depending on its crystallinity degree) which facilitates its shaping (for example by extrusion) and the assembling of parts by welding; it further has a very good chemical resistance to many agents, as well as an excellent behaviour to heat. However, its major defect, due to its crystalline nature, is its high rigidity, which is a bar to its use in a flexible structure.
In the prior art, attempts have been made to cope with this disadvantage by various methods of internal plasticizing (i.e. by copolymerzing vinylidene fluoride with other monomers imparting more flexibility) or external plasticizing (i.e. by incorporating to the polymer various compounds acting as plasticizers).
Whereas the internal plasticizing of polyvinylidene fluoride has been widely studied, it must be observed that it produces a deep disorder in the structure of the homopolymer, which may have an important effect on its properties (for example a decrease of the melting point and of the thermal stability).
As concerns the external plasticizing of polyvinylidene fluoride and also polyvinylidine chloride, the difficulty encountered originates in its highly crystalline nature making imcompatible most of the plasticizers known for polymeric substances produced from vinyl monomers.
Nevertheless, various documents of the prior art relate to external plasticizing of polyvinylidene fluoride or of copolymers containing vinylidene fluoride with other halogeno-olefinic monomers.
For example, U.S. Pat. No. 2,820,772 discloses mainly the plasticizing of trifluorochloroethylene polymers and also that of polymers of vinylidene fluoride with trifluorochloroethylene telomers.
According to U.S. Pat. No. 2,884,399, non-crystalline copolymers of trifluorochloroethylene with vinylidene fluoride (said copolymers containing at least 20% by moles of trifluorochloroethylene), in particular, are plasticized by means of a tetraester formed between a dihydroxylated compound, a dicarboxylic acid and a monohydroxylated compound.
In other respects, U.S. Pat. No. 3,541,039 discloses the plasticizing of polyvinylidene fluoride by polyesters formed between a linear aliphatic dicarboxylic acid of 4 to 8 carbon atoms and an aliphatic diol of 4 to 7 carbon atoms. Specific products are poly(2,2-dimethylpropylene sebacate) and poly(1-methyl propylene adipate).
French patent application No. 2,243,970 discloses the use of acetyl polyesters (for example an acetate of adipic acid-propylene glycol polyester) for plasticizing a polyvinylidene fluoride. Adipic acid-propyleneglycol polyesters are also used to plasticize polyvinylidene fluoride according to Japanese patent application No. 80/84 413. Similarly in Japanese patent application No. 81/20 610, an acetyl polyester is used, for example an acetate of poly(adipic acidpropyleneglycol) to plasticize a mixture of polyvinylidene fluoride with a polymer of methyl acrylate (methyl polyacrylate or isobutylene-methyl acrylate copolymer).
Finally, mention is also found in the prior art of the improvement obtained by using fluorinated resins polyvinylidene fluoride, polychlorotrifluoroethylene) with plasticizers of fluoroalkylphosphate structure (Japanese patent No. 77/73,956) as well as the use of cyanoethyl ethers for decreasing the value of the elasticity modulus polyvinylidene fluoride (Japanese patent application No. 82/187,344).